# materials that interfere with RF

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#### JohnCoker

##### Well-Known Member
OK, so what is it exactly that attenuates RF? Let's say I want an electronics bay with a threaded rod spine, but don't want that spine to attenuate the signal from a parallel antenna.

Presumably steel and aluminum attenuate RF because of their conductivity. Presumably Nylon would not for the same reason. How about titanium (which has 3% the conductivity of copper)?
https://www.mcmaster.com/catalog/126/3366

#### jderimig

The occasional threaded rod in the av-bay doesn't attentuate RF. At worse it may detune the transmitting antenna a little if its very close and the right length.

#### John Kemker

##### Well-Known Member
TRF Supporter
Carbon fiber will attenuate RF. I would not consider putting a transmitter inside a carbon fiber airframe/nosecone without routing the antenna outside.

In addition, certain fillers/pigments/etc in fiberglass can attenuate RF. Think about the zinc used to give epoxy some UV protection, or the metal flakes in paint/gel coat.

As John D. mentioned, the threaded rod wouldn't necessarily attenuate, but instead either detune, giving you a less-than-optimal match to your antenna, or cause directivity issues, giving you more gain in one direction and less in another. The classic Yagi-Uda antenna is an application of this phenomenon in order to create a directive antenna system. It is most likely to cause problems if it's resonant at the frequency of the radio. So, if I were using a 70cm Eggfinder TRS and placed a threaded rod that is 1/4 wavelength of my frequency parallel to my antenna, I would expect issues. However, something that is either longer or shorter by a significant amount, I wouldn't worry as much about.

#### g.pitts

##### Well-Known Member
TRF Supporter
OK, so what is it exactly that attenuates RF? Let's say I want an electronics bay with a threaded rod spine, but don't want that spine to attenuate the signal from a parallel antenna.

Presumably steel and aluminum attenuate RF because of their conductivity. Presumably Nylon would not for the same reason. How about titanium (which has 3% the conductivity of copper)?
https://www.mcmaster.com/catalog/126/3366
That's the answer I selected for the electronics bay I'm designing for my L2 certification (includes a tracker).

#### Voyager1

##### Well-Known Member
TRF Supporter
The main issues with the proximity of metal threaded rods to a typical monopole antenna, as pointed out above by the two Johns, are the effect it has on the antenna impedance and directivity. Changes in antenna impedance at the operational frequency will detune the antenna leading to poor SWR. Changes in directivity will distort the radiation pattern such that in some directions the antenna will be less efficient.

To illustrate the effect on directivity, the two plots below show the simulated radiation patterns of a 1/4 wave monopole antenna (wire 1, in the top plot) in free space and also placed parallel to two 8" x 1/4" threaded metal rods (wires 2 and 3, in the bottom plot). Assume that the rods are below the lower surface of the avionics sled separated by 3", and the monopole antenna is above the upper surface of the sled towards the forward end, centered between the rods. The +Y axis is out from the top of the sled.

Notice the distorted lobe in the 2nd plot. This is due to the proximity of the two rods (wires 2 and 3). Changing the relative position of the antenna wrt the rods will change the shape of the rear lobe (-Y axis).

While I don't consider this to be a definitive description of the effects of metal rods parallel to an antenna, it does illustrate that you might have to think twice about the best location for an antenna. I will see if I can also perform some tests to determine how this issue effects the antenna impedance and resonant frequency.

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#### Voyager1

##### Well-Known Member
TRF Supporter
After performing some simple tests with my NanoVNA (a small and cheap vector network analyser) with a short monopole antenna, it is apparent the the proximity of the threaded rods does have a significant effect on the antenna impedance and resonant frequency. Unfortunately, I didn't have a 900 MHz band monopole with the right connector, so I made these measurements with a 400 MHz band monopole.

Displayed in the following images are: (1) The monopole antenna connected to the VNA; (2) The antenna with parallel Brass tubes, similar to the ones attached to the bottom of the avionics sled; (3) The antenna with parallel threaded Steel rods, typically used in avbays.

In image 1 the VNA measures the resonant frequency of the monopole as approximately 388 MHz and the impedance approximately 49 Ohms after settling. In image 2 the resonant frequency is approximately 404 MHz and the impedance is approximately 58 Ohms. In image 3 the resonant frequency is also 404 MHz and the impedance is approximately 62 Ohms.

While these results are only very rough, they do indicate the potential effects of the parallel metal objects on resonant frequency and impedance. These effects, and those in the previous post, are enough to cause me to stick with threaded Nylon rods in my avbays.

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#### dhbarr

##### Amateur Professional
Thanks for this testing & analysis!

#### timbucktoo

##### Well-Known Member
Staff member
Global Mod
I’d be curious to see same test with the wire whip used on telemetrum or egg products.

#### jderimig

Very nice measurements Voyager! The next step would be to determine what effect there is on radiated field strength. I suspect those would be harder to discern a difference.

Those resonant frequency detunings are surprising to me. I would have expected more. What you have measured are very modest shifts. I suspect that on my of our popular RF products in rocketry that the antenna tune out of the box is much greater than what you measured.

#### gtg738w

##### FlightSketch - flightsketch.com
As noted above, things that are conductive don't really attenuate the signal but they are reflective. At worst case, a reflection perfectly out of phase with the signal could cancel it completely (like noise canceling headphones) but typically it just shows up as more in-band noise. Imagine having a conversation in a room filled with echos. The more noise there is, the harder it is to understand and turning up the transmit power just makes the echos louder too. The same is true for buildings or even the ground. That is why line of sight is so important for range.

#### cerving

##### Owner, Eggtimer Rocketry
TRF Supporter
I’d be curious to see same test with the wire whip used on telemetrum or egg products.
Wire whips are definitely not perfect, but they're easy to make and "good enough" for most purposes. For maximum performance, a properly tuned antenna is best... that's why we include the Linx antennas with the 70 cm Ham products. They cost more, but they have predictable SWR and gain. You can still mess them up by placing them next to allthreads, though... as Voyager1 found, the farther away you keep them from metal, the better.

#### caveduck

##### semi old rocketeer
Thanks Voyager1 for some nice data!! One more thing about orientation - here is the pattern for a pretty nice commercial monopole at 900 MHz:

In this diagram, the radial angles are the distance away from straight out through the antenna axis. Notice that nearly all the pattern goes in the direction the monopole is pointing (this varies by how the ground plane is set up). This means you really, really want the antenna pointing *down* after apogee deployment. I've seen flights where the GPS telemetry disappears after ejection but the rocket is recovered intact with the TX still operating; in most such cases the antenna was pointing the wrong way when under drogue.

#### OverTheTop

##### Well-Known Member
Hey Caveduck. Would you please post a link to that antenna? I have never seen a monopole with radiation in the direction the element is pointing. Thanks.

#### John Kemker

##### Well-Known Member
TRF Supporter
Hey Caveduck. Would you please post a link to that antenna? I have never seen a monopole with radiation in the direction the element is pointing. Thanks.
I think that's a horizontal projection, ie: as seen from the side. Most monopoles (verticals) have similar patterns.

#### Voyager1

##### Well-Known Member
TRF Supporter
Very nice measurements Voyager! The next step would be to determine what effect there is on radiated field strength. I suspect those would be harder to discern a difference.

Those resonant frequency detunings are surprising to me. I would have expected more. What you have measured are very modest shifts. I suspect that on my of our popular RF products in rocketry that the antenna tune out of the box is much greater than what you measured.
Thanks John. This is hardly a rigorous test, though! My intention was to simply illustrate that things do change when metal objects come in close proximity to an antenna.

I agree that the radiated field strength would be more challenging to measure a significant difference. Unfortunately, in retirement, I now don’t have access to an anechoic chamber for far-field measurements.

#### ksaves2

This thread is great as I've been burned with attenuation issues. The major problem that I had in the past was with the metallic paints and GPS trackers in the 70 cm band. Launched two Rockets on the same day, lost 1 and barely found the other 1.
The paint was rattle can Rustoleum metallic. I simply will not fly a metallic painted rocket with a tracker sealed up inside. I have a tendency to use nosecone mounted trackers or an ebay where the antenna is on a bulkhead that is exposed after the apogee charge blows. I do have a Wildman Jr. I fly with a wire antenna stented in the upper, main parachute bay that has non-metallic paint on it. 12 of 13 flights completely out of sight with recovery every time on a Beeline 70cm GPS tracker. I do not know how the 900 MHz trackers will respond to metallic paint nor do I want to risk a rocket to find out. My first misadventure would have been avoided by an adequate ground test which I obviously did not do. Kurt

#### Jay Dub 4009

##### Well-Known Member
Anyone have experience with silver paint on the nose cone and having a feather weight tracker in the nose cone? I want to put a featherweight in the iris I am building but am concerned about it

#### Jay Dub 4009

##### Well-Known Member
Anyone have experience with silver paint on the nose cone and having a feather weight tracker in the nose cone? I want to put a featherweight in the iris I am building but am concerned about it

#### John Kemker

##### Well-Known Member
TRF Supporter
Anyone have experience with silver paint on the nose cone and having a feather weight tracker in the nose cone? I want to put a featherweight in the iris I am building but am concerned about it
I'd recommend against it for the reasons stated above.

#### Jay Dub 4009

##### Well-Known Member
Yes I think the best thing would be to find a light grey replacement for the silver

#### OverTheTop

##### Well-Known Member
My Nike Apache had the NC painted with a metallic aluminium paint. Telemetry good to 37500' on that flight. That's with a TeleMega running 435MHz.

Attenuation could be very paint and process dependent.

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#### AeroAggie

##### Well-Known Member
TRF Supporter
Anyone have experience with silver paint on the nose cone and having a feather weight tracker in the nose cone? I want to put a featherweight in the iris I am building but am concerned about it
The Createx 4300 series metallics use non-metallic things like mica for the reflective bits and should be* completely RF transparent. I say should because I haven't tested it, but my info comes directly from Createx Tech Support.

Stay away from the Createx 4100 series - that uses actual aluminum flakes.